In general, a rolling bearing exhibits a remarkably shortened life when it operates in the presence of foreign matters in its lubricant as compared with when it operates free from foreign matters. This is because if a rolling bearing is lubricated with a lubricant contaminated with foreign matters such as metal cuttings, shavings, burr and powder from abrasion, these foreign matters cause the occurrence of impressions or pits (damage) on the surface of the track on the inner and outer rings and the rolling surface of the rolling elements from which damaged points flaking starts to occur to remarkably shorten the life of the rolling bearing.
An experimental study of this phenomenon, "Contamination with foreign matters and rolling fatigue life", NSK Technical Journal No. 655, pp. 17-24, 1993, discloses that some content of foreign matters, some hardness of foreign matters or some size of foreign matters cause the life of a rolling bearing to be shortened to about 1/8 of that of lubrication free of foreign matters. A reproduction was made of a phenomenon that the effect of contamination with foreign matters and rolling fatigue causes the occurrence of fine impressions having a depth of from scores of micrometers to 100 micrometers on the track or rolling surface of the bearing from which impressions flaking starts to occur, reducing the rolling fatigue of the bearing.
As an prior art approach for eliminating the shortening of rolling life due to this phenomenon there is disclosed in JP-B-7-110988 (The term "JP-B" as used herein means an "examined Japanese patent publication") a technique involving the enhancement of surface hardness of at least one the inner ring, outer ring and rolling elements of a rolling bearing by the formation of a proper amount of fine carbide on the surface layer thereof as well as the elimination of the occurrence of microcracks during lubrication in the presence of foreign matters by the incorporation of a proper amount of retained austenite in the surface layer thereof.
JP-A-9-53169 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses as an alloy composition (hereinafter % by weight) of case hardening steel to be used for carburized parts for driving shaft coupling which must exhibit a high surface hardness an alloy comprising C in an amount of from 0.1 to 0.25%, Si in an amount of from 0.2 to 0.4%, Mn in an amount of from 0.3 to 0.9%, Cr in an amount of from 0.5 to 0.9%, and at least one of Ni in an amount of from 0.3 to 4.0%, Ti in an amount of from 0.01 to 0.3%, Nb in an amount of from 0.01 to 0.3%, V in an amount of from 0.01 to 0.3% and Zr in an amount of from 0.01 to 0.3% and having a surface hardness Hv of from 650 to 800.
Further, Kobe Seiko Giho (Kobe Steel, Ltd.'s Technical Report), vol. 47, No. 2, September 1998, describes in the paragraph titled "Corrosion fatigue characteristics and improvement of steel for high strength suspension spring" an attempt to improve the corrosion fatigue characteristics of spring steel according to JIS SUP7 (C: 0.42%; Si: 1.69%; Mn: 0.18%) from the standpoint of elimination of corrosion pits by the adjustment of components by the incorporation of Cu, Ni, Cr, V and Ti in an amount of 0.20%, 0.33%, 1.06%, 0.15% and 0.05%, respectively, and a material designed taking into account the susceptibility to hydrogen embrittlement.
The rolling bearing disclosed in the above cited JP-B-7-110988 is made of an alloy steel comprising at least C and Cr in an amount of from 0.3 to 0.5% by weight and from 3 to 14% by weight, respectively, and its surface layer has a fine carbide content of from 20 to 50 vol-% and a retained austenite content of from 10 to 25 vol-%. Therefore, the foregoing rolling bearing is advantageous in that it exhibits an enhanced surface hardness and hence an improved impression resistance. However, the foregoing rolling bearing comprises a fine carbide having an average particle diameter as large as from 0.5 to 1.0 .mu.m and thus leaves something to be desired in abrasion resistance.
Further, the case hardening steel disclosed in the above cited JP-A-9-53169 simply comprises alloying elements such as Ti and V incorporated therein to exhibit an enhanced impact resistance or toughness itself. However, some combination of these additive elements cannot be expected to allow the production of fine carbides. Further, when the case hardening having such a composition is used as a material of parts constituting the rolling bearing, impressions can easily occur on the track if the track is contaminated by foreign matters, making it difficult to sufficiently inhibit early flaking. Thus, the foregoing case hardening steel leaves something to be desired in this respect.
Moreover, the high strength spring steel reported in the above cited Kobe Steel, Ltd.'s Technical Report comprises valuable elements such as Cu, Ni, Cr, V and Ti incorporated therein to eliminate the occurrence of corrosion pits from which corrosion fatigue starts to occur. The foregoing high strength spring steel comprises crystals having a reduced size to exhibit an enhanced toughness. However, these ordinary spring steel materials to be subjected to 850.degree. C. oil hardening and 450.degree. C. tempering cannot be expected to exhibit a surface hardness which can sufficiently satisfy impression resistance or abrasion resistance required for rolling bearing.
On the other hand, as an example of the shortening of the life of a rolling bearing by contamination by water there is described in J. A. Cirura et al., "Wear, 24 (1973), pp. 107-118, The Effect of Hydrogen on the Rolling Contact Fatigue Life of AISI 52100 and 440C Steel Balls" a four-ball rolling test involving the contamination of the lubricant with water resulting in the shortening of the life of a rolling bearing to about 1/10 of that free of water. This reference also describes that a rolling fatigue test on steel balls charged with hydrogen showed that a stainless steel ball has a longer life than a steel ball of the second kind of bearing steel. However, the use of such a stainless steel ball as a countermeasure for the prolongation of life costs much as compared with the use of a steel ball of the second kind of bearing steel and thus can hardly be realized in practice.